4.2.4: Analytical techniques Flashcards

1
Q

What happens when a molecule absorbs infrared radiation?

A

It makes the covalent bond vibrate more

in a stretching or bending motion

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2
Q

What factors affect the amount of vibration of a bond?

A

● Bond strength
● Bond length
● Mass of each atom in the bond

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3
Q

How does infrared spectroscopy work?

A

Every bond has a unique vibration frequency in the infrared region of the EM spectrum
Bonds absorb radiation that has the same frequency as their frequency of vibration
Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group

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4
Q

What do the peaks on an infrared spectrum represent?

A

Absorbance of energy from the infrared

radiation

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5
Q

What are the 2 peaks that must be present to identify a substance as carboxylic acid?

A

● Very broad peak at 2500 - 3300 cm-1 → O-H group

● Sharp peak at 1680 - 1750 cm-1 → C=O group

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6
Q

What happens inside a mass spectrometer?

A

● Organic compound is vaporised and passed through the spectrometer
● Some molecules lose an electron and forms molecular ions
● Excess energy from the ionisation makes the bonds vibrate more
● Vibration causes bond to weaken
● Molecular ion splits by fragmentation

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7
Q

What is the symbol of molecular ion?

A

M+

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8
Q

Is fragmentation predictable? Why?

A

No, because it can happen anywhere in

the molecule

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9
Q

In a mass spectrometry how is a molecular ion represented?

A

It is the peak with the highest mass/charge ratio

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10
Q

The molecular mass of the molecular ions is equal to what?

A

Relative molecular mass of the compound

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11
Q

Will the molecular ion peaks of two isomers of the same compound be same or different?

A

Same

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12
Q

What is the m/z value of CH3+?

A

15

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13
Q

What is the m/z value of OH- from alcohol?

A

17

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14
Q

What is the m/z value of C2H5+?

A

29

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15
Q

What is the m/z value of C3H7+?

A

43

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16
Q

What is the m/z value of C4H9+?

A

57

17
Q

What are the advantages of using mass spectrometry? (2 marks)

A

● Cheap

● Small quantities of samples required

18
Q

What is the main disadvantage of using mass spectrometry?

A

The sample is completely destroyed